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Performance issues with line merged PDF files
Performance issues with line merged PDF files
The process of converting PDFs to TIF or other raster (JPEG, BMP, etc.) formats takes a long time with "Line Merged" PDFs.
The term "Rasterizing" is used to describe the process of converting vector data
(which is the way that line drawing information is typically represented in a
PDF) to bitmapped data (as is typical in a TIF, BMG, or GIF file). Rasterizing
PDF files can take a significant amount of time, especially at high DPI when
working with PDFs that have transparency features enabled.
The process of rasterizing is most commonly encountered in the architectural and
engineering disciplines in PDFs which have "Line Merge" enabled for printing
purposes. Starting with Pushbutton PDF version 2.2.8, Bluebeam added the
capability to create line merged PDF files. Displaying line merged PDF files
results in overlapping entities blending colors together to form new colors
based on the particular blend mode in use. For example, when line merge is
enabled, the color displayed at the intersection point where a blue and yellow
line overlap each other is displayed as a combination of the 2 colors and will
result in a new color.
Rasterizing (converting the vector PDF file to a raster image format) a line
merged PDF file is a computationally intensive process because the color value
of each pixel has to be computed. As the DPI goes up the performance problems
intensify. Take for instance an ARCH_D PDF (36” x 22”) at 400 DPI. That equates
to 36 * 22 * 400 * 400 = 126,720,000 pixels. When line merge is enabled each of
those pixels can possibly be blended with pixels of different layers.
The PDF file specification supports a transparent imaging model which allows
for line merge. However, printers do not support transparency. When printing
PDF files with line merge they must first be flattened to simulate
transparency. For example a common flattening process is to convert the PDF
file to an image. Reprographers often do this as a step in the printing process.
Reprographers often use Ghostscript to do the conversion of PDF files to TIF.
Older versions of Ghostscript (such as 8.14) ignore line merge information in
the PDF, thereby processing the PDF file much faster but without taking into
account blending. Newer versions of Ghostscript (8.5.0 and later) process the
line merge information correctly, but the process takes much longer.
Suggestions for improving performance
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Line merge should only be used when needed. When printing PDF files in black
and white it is often difficult to see visual differences between line merged
and non line merged prints.
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Architects can often eliminate the need for line merge by reordering the layers
in the AutoCAD drawing. Order the layers of the drawing such that the screen or
underlying color is on a lower layer and interior elements are on the top layers.
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Reprographers using Reprodesk Software can easily have side by side
installations of Ghostscript. Ghostscript version 8.14 is the last version that
does not process line merge, version 8.53 is the lastest as of this writing.
8.14 can be used as a fallback for print jobs that would take too long using
8.53. Note that the TIF will not be line-merged if version 8.14 of Ghostscript
is used.
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